Process for the preparation of methyl bromide using radiation

ABSTRACT

THE PRESENT INVENTION IS A PROCESS FOR THE PREPARATION OF METHYL BROMIDE WHICH COMPRISES REFLUXING METHANOL AND BROMINE WITH EXPOSURE OF THE VAPORS TO RADIATION OF SUFFICIENT ENERGY TO ACTIVATE THE BROMINE. THE PROCESS RESULTS IN GREATER THAN 90% OF THE BROMINE EMPLOYED BEING CONVERTED TO METHYL BROMIDE.

Y 3,682,805 PROCESS FOR THE PREPARATION OF METHYL BROMIDE USING RADIATION Arthur A. Asadorian, Midland, and Max R. Broadworth,

Bay City, Mich., assignors to The Dow Chemical Company, Midland, Mich. N Drawing. Filed Sept. 4, 1969, Ser. No. 855,368

Int. Cl. 1501i 1/10; C07c 17/00 [1.8. C]. 204-163 R 9 Claims ABSTRACT OF THE DISCLOSURE The present invention is a process for the preparation of methyl bromide which comprises refluxing methanol and bromine with exposure of the vapors to radiation of sufiicient energy to activate the bromine. The process results in greater than 90% of the bromine employed belng converted to methyl bromide.

BACKGROUND OF THE INVENTION Methyl bromide, a well known refrigerant, fire extinguishing agent and soil fumigant is prepared by the reaction of methanol and hydrobromic acid. In this process, the hydrobromic acid is either generated in situ from bromides and sulfuric acid or externally from bromine and hydrogen. In the former case, sulfuric acid is a necessary constituent of the reaction mass, while in the latter a separate reaction step is involved.

Methyl bromide is also prepared by reacting methanol with a solution of sulfur in bromine. This process produces good yields of the desired product. However, sulfuric acid is produced as a by-product which combines with unreacted sulfur to form a sludge which creates Waste disposal problems.

US. Pat. 2,173,133 discloses that alkyl bromides can be prepared by reacting an alcohol with bromine in the presence of a reducing agent. The preferred reducing agent for this process is S0 which is converted to sulfuric acid during the reaction. The acid concentration must be carefully confined within certain limits. If the acid is too concentrated, the formation of ether can occur, while if the acid is too dilute, bromine in the form of hydrobromic acid is lost in the effiuent sulfuric acid.

German Pat. 703,835 discloses that methyl bromide may be prepared by reacting methyl alcohol and bromine in the presence of a surface active catalyst. This process requires reaction temperatures of 150 C. or higher as well as the use of a catalyst.

Buckley et al. report in Trans. Far. Soc. 55, 1536 (1959) that when methanol and bromine vapors are contacted in a sealed vessel under reduced pressure in the presence of light, carbon monoxide and hydrogen bromide are formed. This reference goes on to state that the formation of some methyl bromide and water occurred after the products were condensed in liquid air. This procedure was reported to produce relatively small yields of methyl bromide, and only when methanol was employed in a molar ratio to bromine of greater than 4:1, did the yield of methyl bromide approach 50 percent. A two stage reaction is reported, represented by the equations:

It would be desirable and it is a principal object of the present invention to provide a novel process for the preparation of methyl bromide.

It is a further object to provide such a process wherein methanol and bromine are reacted without the use of reducing agents or material catalysts.

3,682,805 Patented Aug. 8, 1972 An additional object is to provide such a process in which the formation of undesirable side products, especially those causing waste disposal problems, is minimized or obviated.

An additional object is to provide such a process which will produce high yields of methyl bromide.

SUMMARY, OF THE INVENTION The above objects and other advantages are achieved in the process for the preparation of methyl bromide which is disclosed herein. The present process comprises refluxing a mixture of methanol and bromine with the vapors being subjected to radiation of suflicient energy to activate the bromine.

The term refluxing as used herein is intended to mean heating the methanol and bromine to a temperature sufiicient to vaporize them and then cooling the vapors to a temperature suflicient to cause at least part of the methanol vapor to condense. The process is carried out in a reactor designed to allow carbon monoxide formed in the reaction to escape without causing the pressure to increase substantially above ambient.

DETAILED DESCRIPTION OF THE INVENTION The process of the present invention can be carried out in a reflux condenser having the temperature controlled to reflux methanol and allow the methyl bromide product to pass through as vapor. Use of a fractionating column fitted with a refrigerated total condenser can serve to reflux methanol and allow the methyl bromide prodproduct as condensate.

Bromine and methanol are usually introduced to the apparatus as liquids although they may be prevaporized and fed into the apparatus as gases. Sufiicient heat is applied to the reactants to cause vaporization with the vapors being exposed to radiation of sufiicient energy to activate the bromine, i.e.

The process has been found to operate efiiciently with ultraviolet light as the energy source. However, lower and higher energy radiation may be used provided suflicient energy is supplied to activate the bromine under the reaction conditions. For example gamma radiation and X-rays effectively activate bromine. Lower energy radiation, such as certain wavelengths of visible light up to about 5100 A. will also activate bromine. Radiant energy in the visable range having wavelengths greater than 5100 A. will activate bromine. Weisberger reports in Technique of Organic Chemistry, vol. II, 2nd edition at page 266 that light having wavelengths of from 5107 to 6320 A. will cause the bromine molecule to dissociate. However, the rate of dissociation is reduced at these wavelengths. Therefore, radiant energy having wavelengths in the ultraviolet range is the preferred energy source.

Radiation other than radiant energy, such as alpha and beta particles as well as high energy electrons such as those accelerated by a Van de Graaff generator, may also be used to provide energy for bromine activation.

For most efiicient operation the reflux condenser should be cooled to a temperature which will cause complete condensation of the methanol and bromine vapors under the reaction conditions. A condenser temperature range of about 5 to 15 C. is sufficient for complete condensation at normal atmospheric pressure. With a condenser temperature of below 15 C. and above about 5 C. a mixture of methyl bromide and carbon monoxide, which is coproduced in the reaction, will pass through the condenser. This mixture of gases can readily be separated to provide the desired methyl bromide. The carbon monoxide may be recovered as a useful by-product. With a condenser temperature of greater than 15 C. some methanol will pass through. Hydrogen bromide and water are also produced as evidenced by the formation of hydroratios of methanol and bromine should be used that are stoichiometric for the overall reaction bromic acid in the boiling vessel. Unexpectedly, greater A molar ratio of methanol to bromine of greater than than 98 percent of the bromm? charged can be recpverhd 5:2 will result in a further increase in yields based on in the form of methyl bromide and hydrobromic acid bromine mchcatmg that HBI does not pass thrfmgh the condenser The following examples will serve to further illustrate This occurs even when the condenser is open to the atmosthe invention phere and cooled with ordinary ice water. EXAMPLES An alternative method of recovering the methyl bromide is to insert a distillation column between the boiling In each the following examples a 500 milliliter Vessel and the condenser and 2 the condenser a neck balloon flask, fitted with a bromine feeder, stirrer and temperature e e h below 0 Such as by a cold water reflux condenser, was charged with 3 moles Dry Ice or liquid nitrogen as the coolant. The coolant (96 grams) of methanol and 12 moles (192 grams) of h ld m }o th ond r t a temperature of at E bromine. The bromine and methanol mixture was refluxed about Ih thls embodhhehti carboh mohoxlde for various periods of time up to 6.5 hours. The methyl Passes through the Fehdehser wlth essehhahy ah of 'E bromide vapors passed directly from the reactor through methyl bremlde being The methyl brohhde the reflux condenser to a water scrubber which leached can then be collected and drained off from the bottom out any methanol and then through a caclz drier to a of the condenser. This is normally done when the tem- Dry Ice condenser perature of the vapors in the distillation column approach In Examples where the activation energy was the belhhg Pomt of fhethyhhromlde to ylelh a methyl vided by either a GE. sunlamp or a 100 watt H4AB G.E. bremlde pr'edue't of hlgh P Methyl hremlde may be ultraviolet lamp, average yields of greater than 80% were recovered in this manner until the reaction between the obtained in operating times of less than Six hours bromine and methanol is completed. The reaction has ap- Examples represent comparative experiments E preached eelheletlonvwheh the vapee temperlzmre mes ample 6 illustrates the advantage of using radiation of above The 730111118 Point of methanol higher energy than visible light. When ordinary labora- Ahother method of reeovenng methyl bronhde 15 to tory lighting, which was present during each experiment, cool the fefi'llfilhg P to a tempefamlfe e e was the source of activation energy, methyl bromide was low the holhhg P {hethyl broh'hde to hquefy recovered in a 22.8% yield during a reaction time of 6.5 h thfi feactlon cohdltl'ons Y h low enough to hours. In Example 5, where the bromine and methanol q y the Q P ITlOhOXlde- 'f embodlmem, mixture was not brought to the boiling point of methanol, methyl bromide is chhected as hqllld Condensate Such as no formation of methyl bromide was observed. Sulfur was through a slde Vent 1n the TefluX Column and the carhoh used in Example 7 to check its usefulness as a catalyst. IhOhOXlde allowed t0 escape as a e A 9 8 The yield in this experiment was 54.4% during the six temperature of below about 0 C. is necessary with a hour operating period. This corresponds to the yield pr Cooling range of from about -30 C to about C- dictable when assuming that all of the sulfur would be being preferred for efiicient condensation of the methyl 40 present as SBr and reacts as a reagent as disclosed in bromide. US. Pat. 2,359,828.

TABLE I Example number 1 2 3 4 5 6 7 Radiation source arid/or catalyst employe (a) Sulfur (5 g.). 0 0 O O 0 X (b) sunla i X o o o 0 0 (o) Ultraviolet lamp 0 X X 0 0 O Bromine teed period hrs 0.5 0.3 0.4 4.4 0.3 0.4 Total operating time hrs--. 6. 2.2 0.9 5.6 6.5 6.0 Temperature range, 0 61- -102 54-106 66-105 57-63 59-75 59-80 CHaBr recovered, gins 8 124 Percent yield based on Br: 82.4 82.2 79.0 0 22.8 54.4

l Vigorous reaction rate caused back pressure and some losses.

In each embodiment, carbon monoxide is allowed to escape from the reaction vessel as it is formed. Allowing carbon monoxide to escape is conveniently accomplished by carrying out the reaction in a vessel having at least one opening through which it can vent.

The process may be run in a continuous manner by periodically draining the hydrobromic acid from the boiling vessel and introducing additional methanol and bromine. Alternatively, the process may be carried out batch by batch.

The methyl bromide recovered normally contains small amounts of impurities such as methyl ether, methylal, methyl formate and some unreacted methanol. Except for methyl ether, these impurities are soluble in water and can easily be separated from the water insoluble methyl bromide by water scrubbing. Methyl ether which remains as an impurity can be removed by conventional means such as scrubbing with sulfuric acid.

The molar ratio of methanol to bromine to be employed is not critical. However, for good yields molar EXAMPLE 8 An experiment was conducted in equipment similar to that of Example 1 except that a Vigreaux fractionating column was used for more efficient distillation. The column was fitted with a total condenser cooled with Dry -Ice in CH Cl The product was removed as vapor or liquid condensate at the base of the condenser. The ultraviolet lamp was used as the radiation source.

In one run, three moles (96 grams) of boiling methanol was treated dropwise with 1.5 moles (240- grams) of bromine added over a period of 2.0 hours. After 88 percent of the bromine had been added, the boiling liquor temperature dropped from an initial 65 to 55 C. under total refluxing conditions, and the vapor temperature dropped to 7 C. The product was taken off slowly with the vapor temperature holding at about 7 C. The total product recovered was 208.8 grams of which 196.0 grains was found to be pure methyl bromide. This was an 86 percent yield based on bromine as the limiting reactant.

The total time of the run was 5.6 hours with maximum temperatures being 30 C. for the vapors and 99 C. for the boiling liquor which remained red throughout the entire process.

EXAMPLE 9 In a manner similar to that of Example 8 six moles (192 grams) of boiling methanol was treated dropwise with 1.5 moles (240 grams) of bromine over a period of 1.65 hours. After all the bromine was added, the boiling liquor temperature dropped to 62 C. and the vapor temperature to 8 C. under total refluxing conditions. After an additional 1.5 hours, with the red boiling liquor still at 62 C. and the column vapors at 8 0., product takeoff was started at a controlled rate to maintain the vapor temperature slightly above the boiling point of methyl bromide. In an additional 1.3 hours the boiling red liquor had dropped to 57 0., its minimum for the entire run. After a total time of 11.5 hours, the boiling liquor became light yellow at 80 C. and the vapor reached its maximum of 30 C.

The recovered product weighed 265.8 grams, of which 240.4 grams were pure methyl bromide. This was an 84.3 percent yield based on bromine as the limiting reactant.

EXAMPLE 10 Three moles (96 grams) of methanol was treated dropwise with 1.2 moles (192 grams) of bromine which provided proportions stoichiometric for the reaction The bromine was added over a period of 4.1 hours While the liquor was being refluxed using an ice water, C., condenser. Some of the methyl bromide formed vented off with the carbon monoxide through the ice water condenser and was collected in a Dry Ice trap.

Product takeofi was commenced when the vapor temperature reached 8 C. After an operating time of 6.7 hours a total of 212.3 grams of product was recovered from the cold trap and product takeoff. This represented a 93.2% yield of product which mass spectral analysis disclosed to consist of:

Percent Methyl bromide 97.3 Methyl ether 0.18 Methylal 0.34 Methyl formate 1.46 Methanol "h 0.7

Based on this analysis, the yield of pure methyl bromide was 90.6%.

The residual hydrobrornic acid in the reactor pot weighed 56.1 grams and contained 15.8 grams of BF" or 8.2% of the feed bromine. This plus the 90.6% bromine accounted for as methyl bromide gave a total of 98.8% of the feed bromine accounted for.

EXAMPLE 11 An experiment was conducted in a manner similar to Example except that the condenser was cooled with Dry Ice in CHgClg. All of the methyl bromide was recovered by product takeoif since it could not pass through the Dry Ice condenser. Pure methyl bromide was recovered in a yield of 90.5%. The bromine recovery was determined to be 98.7%.

EXAMPLE 12 An additional experiment was conducted in the manner of Example 11 except that a 200 watt, 120 volt lamp was used as the radiation source rather than the ultraviolet light as in Example 11. During a total reaction time of 7.6 hours, 199.2 grams of 95% pure methyl bromide was recovered. This represented a methyl bromide yield of 83% based on the bromine fed.

We claim:

1. A process for the preparation of methyl bromide which comprises:

(a) refluxing, in a vessel having at least one opening for vent gases, a mixture of methanol and bromine while exposing the vapors to radiation of sufiicient energy to activate the bromine thereby forming a reaction product containing methyl bromide and carbon monoxide, and

(b) allowing the carbon monoxide to escape through the opening as a vent gas.

2. The process of claim 1 wherein the radiation is in the form of ultraviolet light.

3. The process of claim 1 wherein the refluxing vapors are cooled to a temperature of from 5 to 15 C.

4. The process of claim 1 wherein the methanol and bromine are condensed at a temperature sufficiently high to permit at least some of the methyl bromide product to pass through the reflux condenser and the product which passes through is condensed at a temperature of at least about 30 C.

5. The process of claim 1 wherein the refluxing vapors are cooled to a temperature below the boiling point of methyl bromide but above the boiling point of carbon monoxide thereby permitting carbon monoxide to escape as a vent gas and collecting methyl bromide as a liquid condensate.

6. The process of claim 5 wherein the refluxing vapors are cooled to a temperature of from about 30 to about C.

7. The process of claim 1 wherein the molar ratio of methanol to bromine is at least 5:2.

8. The process of claim 1 wherein the methyl bromide product is purified by water scrubbing.

9. The process of claim 8 wherein the product is scrubbed with sulfuric acid after water scrubbing.

References Cited UNITED STATES PATENTS 2,540,127 2/1951 Lacomble et a1 204l63 2,639,301 5/1953 Ruh et a1. 204-163 X 3,428,539 2/ 1969 Lester 204-16 3 FOREIGN PATENTS 791,702 8/1968 Canada 204-l63 BENJAMIN R. PADGE'IT, Primary Examiner US. Cl. X.R. 204163 HE UNHED STATES PATENT @FFE'QE QARTWMIATA @5 (JQRREQTlGN Patent No. 3,682,805 Dated August 8, 1972 lnventofls) A. A. Asadorian & M. R. Broadworth It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2 line 30, delete "methanol and allow the and "prod-" and insert after reflux -all of the alcohol and take off--.

Signed and sealed this 29th day of May 1973.

(SEAL) Attest: v

EDWARD M.FLETCHER,JR ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents UNHED STATES PATENT @FFE'QE QARTWMIATA @5 (JQRREQTlGN Patent No. 3,682,805 Dated August 8, 1972 lnventofls) A. A. Asadorian & M. R. Broadworth It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2 line 30, delete "methanol and allow the and "prod-" and insert after reflux -all of the alcohol and take off--.

Signed and sealed this 29th day of May 1973.

(SEAL) Attest: v

EDWARD M.FLETCHER,JR ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents UNK'EED STATES PATENT @FFKCE tohmtm QRTWEQATE Patent NO. Dated A t 8, Inventor) A. A. Asadorian & M. R. Broadwo rth It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 30, delete "methanol and allow the" and "prod-" and insert after reflux -all-of the alcohol and take off-.

Signed and sealed this 29th day of May 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

